`Background Noise' in Electromyography

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Abstract
Small electrical fluctuations were detected during electromyographic investigations of postural muscles in human subjects at rest. These fluctuations might be caused by thermal noise in the source impedance. The impedance of the electrodes and tissues was measured and it was found to contain ` polarization elements '. In supplementary experiments a metal-electrolyte polarization cell was found to generate the same thermal noise as its equivalent impedance at a given frequency. It is concluded that it is reasonable to expect that the tissues and electrodes will generate the thermal noise which can be calculated from the values of their measured impedance. With electrodes attached to resting subjects, the fluctuations were found to be greater than the noise from the equivalent impedance. The spectra of the excess fluctuations, and of the electromyograms of contracted muscles, were obtained. Possible sources of the fluctuations are suggested. It is unlikely that they are caused by the intermittent activity of muscle fibres. This is of interest in relation to hypotheses of muscular tone. The fluctuations represent the lower limit of the signal which must be accommodated in electromyographic studies using surface electrodes and set an upper limit to the useful sensitivity of the amplifier.